System and method for intensity monitoring

ABSTRACT

A detection system for monitoring the intensity of a stream of modulated pulses, the system comprising a controller configured to provide a control signal to a component outputting modulated pulses the control signal controlling the level of modulation for each pulse exiting the component and a detector configured to measure the intensity of the pulses in the stream of pulses exiting the component outputting modulated pulses, wherein the detector comprises a gated detector, the controller being configured to send a gating signal to said detector, wherein said gating signal varies the gain of the detector with the control signal, such that pulses with a selected modulation level can be detected.

FIELD

Embodiments described herein relate generally to a system and method forintensity monitoring.

BACKGROUND

Many communication systems such as quantum communication systemstransmit information using a high frequency pulse train, for example,frequencies in excess of 10⁸ pulses per second. In some such systems,the pulses are transmitted with different intensities for example, tomonitor security. By varying the intensity of the pulses in a quantumcommunication system, it is possible to monitor the security of thesystem by determining the number of pulses of different intensitieswhich were successfully transmitted. Such a method requires goodknowledge of the intensity of the high frequency pulses sent.

The intensity of such pulses is typically varied by an intensitymodulator. However, the performance of such modulators tends to varyover time and is very susceptible to temperature fluctuations.

Also, other modulators such as phase and polarisation modulators areused to modulate high frequency pulse trains. Although such modulatorsdo not attempt to control the intensity of the pulses, they mayunintentionally vary the intensity during modulation of thepolarisation/phase etc.

DETAILED DESCRIPTION

FIG. 1 is a schematic of a quantum communication system;

FIG. 2 is a schematic of a sending unit incorporating a detection systemin accordance with an embodiment of the present invention;

FIG. 3 is a plot of a pulse train of the modulator of FIG. 2;

FIG. 4 is a schematic of a sending unit with a modulator used forbit/basis selection in accordance with an embodiment of the presentinvention; and

FIG. 5 is a is a schematic of a sending unit incorporating a detectionsystem in accordance with a further embodiment of the present invention.

In an embodiment, a detection system for monitoring the intensity of astream of modulated pulses is provided, the system comprising acontroller configured to provide a control signal to a componentoutputting modulated pulses the control signal controlling the level ofmodulation for each pulse exiting the component and a detectorconfigured to measure the intensity of the pulses in the stream ofpulses exiting the component outputting modulated pulses, wherein thedetector comprises a gated detector, the controller being configured tosend a gating signal to said detector, wherein said gating signal variesthe gain of the detector with the control signal, such that pulses witha selected modulation level can be detected.

The above system allows the intensity of a stream of pulses to beaccurately measured on a pulse by pulse basis.

In a further embodiment, the controller is configured to group thepulses into frames with each frame comprising a plurality of pulses, thecontroller being configured to control the gain of the detector todetect pulses modulated by a selected control signal.

In one example, the controller is configured to control the component tovary the modulation between J different levels where J is an integer ofat least 1, the controller being configured to control the detector tomeasure over at least J frames, such that the gain of the detector isset to detect pulses of intensity level J for each of the J frames. In afurther embodiment, the the gain of the detector is switched to a levelnot to detect pulses for at least one additional frame in order tomeasure the dark count level.

In an embodiment, the output of the detector is integrated over eachframe. However, as the detector is gated with the control signal itsgain can be increased to a detection level only for the pulses with theselected modulation within an integration period.

In one embodiment, the detector is an avalanche photodiode.

In an embodiment, the system works with a very high frequency source,for example, in excess of 10⁶ pulses per second. In other embodiments atleast 10⁸ pulses per second or at least 10⁹ pulses per second. In someembodiments, the frame will last 1 second, with the sender sending at arate of at least 10⁶ pulses per second. In other embodiments there willbe at least 10² or 10³ pulses per second. In further embodiments, longeror shorter frame lengths may be used.

The system may be configured to provide feedback from the intensitymeasurement to the control signal to correct for drifting intensities.For example, the controller may be configured to vary the control signalsent to the modulating component dependent on the measured intensities.

The detection system may be used in the sender of a quantumcommunication system. In some cases the component outputting modulatedpulses is an intensity modulator. However, the component may be anothertype of modulator, for example a phase or polarisation modulator. Suchmodulators may unintentionally affect the intensity and the system canbe used to detect for variations in the intensity when differentmodulation signals are applied, even though these modulation controlsignals are not intended to vary the intensity.

In a further embodiment the modulated pulses are provided by a variableintensity source, for example a pulsed laser where the intensity of theoutput pulses is dependent on an applied voltage and the controller isconfigured to control that voltage. In a further embodiment, a variableintensity source is provided by a plurality of light sources where eachsource is of a different intensity and the outputs of the sources arecombined such that the intensity of the output can be controlled byselecting a particular source, the source may be selected using thecontrol signal from the controller.

In further embodiment, a detection system for monitoring the intensityof a stream of modulated pulses is provided, the system comprising acontroller configured to provide a control signal to a componentoutputting modulated pulses and a detector configured to measure theintensity of the pulses in the stream of pulses exiting the componentoutputting modulated pulses, the controller being configured to groupthe pulses into frames and control the detector to measure the averageintensity of each frame, the controller being configured to apply acontrol signal selected from J different control signals where J is aninteger of at least 1 and vary the distribution of the pulses betweenthe frames such that J frames can be produced with a differentdistribution pulses subjected to the control signals in each frame, thecontroller being configured to associate the distribution of pulses withthe detected average intensity value for each frame.

In an embodiment, the controller further comprising a calculating unit,the calculating unit being configured to receive information concerningthe distribution of pulses in a frame with the detected averageintensity for each frame and calculate the intensity of the pulsesexiting the modulating component for each control signal applied by thecontroller from:

${\langle I\rangle}_{j} = {I_{dc} + {v\; q{\sum\limits_{j = 0}^{J}{u_{j}{p\left( u_{j} \right)}}}}}$

Where <I>j is the average intensity over a frame of pulses, I_(dc) isthe dark count current; v is the repetition rate of the light pulses; qis the “detector gain”, J is the total number of levels of controlsignals applied to the modulating component, u_(j) is the intensity ofthe pulse exiting the modulating component controlled by signal of levelj; p(u_(j)) is the probability that level j was applied to the modulatorwhen the pulse passed through the modulating component.

In an embodiment, a method of monitoring the intensity of a stream ofmodulated pulses is provided, the method comprising:

-   -   outputting a stream of modulated pulses from a component;    -   providing a control signal to said component to modulate said        pulses, the control signal controlling the level of modulation        applied to each pulse by the component; and    -   measuring the intensity of the pulses exiting the component        using a gated detector, said detector being controlled using a        gating signal which varies the gain of the detector such that        pulses with a selected modulation level can be detected.

In an embodiment, a method of monitoring the intensity of a stream ofmodulated pulses is provided, the method comprising:

-   -   outputting a stream of modulated pulses from a component;    -   providing a control signal to said component to modulate said        pulses, the control signal selected from J different modulation        levels where J is an integer of at least 1;    -   grouping the pulses into J frames wherein the distribution of        the pulses with different modulation levels is varied between        the frames; and    -   measuring the average intensity of each of the frames.

Methods in accordance with embodiments of the present invention can beimplemented either in hardware or on software in a general purposecomputer. Further methods in accordance with embodiments of the presentcan be implemented in a combination of hardware and software. Methods inaccordance with embodiments of the present invention can also beimplemented by a single processing apparatus or a distributed network ofprocessing apparatuses.

FIG. 1 is a schematic of a basic quantum communication system where asender Alice 1 sends a message to a receiver Bob 3 over a quantumcommunication channel 5. Alice 1 and Bob 3 can also communication over aclassical channel 7. A third unauthorised party, called Eve 9, wants toeavesdrop on what they are saying.

FIG. 2 is a sender comprising an intensity modulator in accordance withan embodiment of the present invention. The sender 11 comprises a source13 which in this case is a pulsed laser. However, the source could be adedicated single photon source which is optically or electricallydriven.

The pulsed laser produces a train of regular pulses. In someembodiments, a high frequency pulsed laser is used which emits pulses atspeeds of 10⁸ pulses per second or higher.

The sender 11 further comprises an attenuator (not shown) whichattenuates the output of the pulsed laser. Before the attenuator, thepulses have an average number of photon per pulse equal to u_(a). Theattenuator decreases such intensity by several orders of magnitude, tothe new level u_(b). This is done to approach the quantum regime, whichrequires having only a small number of photons in each pulse. In anembodiment a typical value of u_(a) can be 10⁸ while u_(b) is about 1.The attenuator does not necessarily precede the other parts of thesender 11 and actually in some embodiments it is put after othercomponents in the sender 11.

Next, the pulses enter the detection system block 15. In an embodiment,the attenuated pulses go through an intensity varying component 17 wheretheir intensity is further modified. In this particular embodiment, themodulator is an intensity modulator which modulates the intensity of astream of pulses. However, other components which can modulate pulsesmay be used. For example, the component 17 may a pulsed laser sourcewhich may receive a control signal to vary the intensity of pulsesoutput by the laser. Alternatively, the laser source may be a pluralityof pulsed laser sources and the component is configured to selectbetween the sources. The component may also be a modulator or othercomponent which receives control signals to vary some attribute of thepulses other than intensity, but where the intensity may beunintentionally modulated as well.

The component 17 can set the average number of photons per pulse to avery precise level, as required by the particular application. Forexample, the component 17 can prepare three states whose average photonnumbers are u₀, u₁ and u₂. In an embodiment 3 states are prepared.However, a different number of states can be used. In an embodiment, ifthe three states are used, they can be set the following values:u₀=10⁻⁴, u₁=0.1 and u₂=0.5; but different values can be used.

The generic photon number value of a particular state is denoted asu_(j), where j={0, 1, 2, 3, . . . }. The states u_(j) can be preparedwith different probabilities p(u_(j)) by the component 17. It is notrequired that they are prepared with the same probability. For instancethe state u₂ can be prepared with a probability of 90%, u₁ with aprobability of 9% and u₀ with a probability of 1%.

In the embodiment described in relation to FIG. 2, the action of thecomponent 17 is verified using detector 19 to ensure that the user knowsthe actual intensity of photons leaving the sending unit 11.

The component is controlled by controller/processor 21. In order toprepare the intensity modulation pattern showed in the figure, given by(u₂, u₂, u₁, u₂, u₀, u₂), the controller 21 will generate a voltagesignal with the same pattern and will apply it to the component 17.

The output of the component 17 is then passed into beam splitter 23. Inthis embodiment, beam splitter 23 is a 50:50 beam splitter such that 50%of the incoming light passes through beam splitter 23 into the quantumchannel and 50% is directed into detector 19. However, beams splitterswith other ratios can be used. If the attenuator (not shown) is providedbefore the beam splitter, the attenuation provided by the attenuatorwill take into account the effect of the beam splitter. For example, ifthe pulses are to be attenuated to a level u_(j) for entering thequantum channel, then the attenuator will attenuate the pulses to 2u_(j)and the beam splitter 23 (if a 50:50 splitter) will reduce the intensityfurther to u_(j).

The beam splitter 23 then divides the pulse train and directs part of itto the quantum channel and part to detector 19. Detector 19 is a GatedDetector. This detector has the peculiarity that its response depends onan external voltage that may be applied on it. A typical example ofGated Detector is the Avalanche Photo Diode (APD) detector. In the APD,an incoming photon is initially converted into an electron-hole pair.The charges are accelerated by an electric field and generate anavalanche through the medium by impact ionisation. The secondary chargesgenerated during the avalanche give rise to a current which isproportional to the number of incident photons. By measuring thiscurrent, the total number of impinging photons can then be known. Theproportionality coefficient between the number of incident photons andthe detected current is the “gain”. The APD gain can be varied in theapproximate range 1÷1000 by acting on the electric field responsible forthe avalanche. An experimenter can control the electric field applied onthe APD and thus vary its gain. The working principle of the APD iscommon to many Gated Detectors. The gain can be controlled between ahigh level (q_(high)) where the detector detects photons and a low level(q_(low)) where the detector does not detect photons.

In the system of FIG. 2, there are 3 examples of rapidly varyingelectric fields, depicted by square shaped patterns beside the labels“q₂”, “q₁” and “q₀”. Such patterns are chosen accordingly to measure theclass j of the light pulse.

In the embodiment of FIG. 2, a modulation pattern is sent to thedetector 19 from the controller 21 similar to the intensity modulationpattern sent to the component 17 with the purpose of changing thedetector gain, in a way that is correlated with the intensity modulator.

FIG. 3 shows a possible way of modulating the gain in accordance with anembodiment of the invention. Here:

-   -   a first pattern is prepared to measure the class j=2. The gain        of the Gated Detector is set to ^(“q) _(high)” in correspondence        of those time slots in which the intensity modulation pattern        presents a value u₂. In FIGS. 2 and 3 this happens in        correspondence of the 1^(st), 2^(nd), 4^(th) and 6^(th) time        slot. In all the other time slots the gain is set equal to a        reference value that shall be denoted as “q_(low)”. As an        example, q_(high)˜1000 and q_(high)˜1.    -   A second pattern is prepared to measure the class j=1. The        voltage on the Gated Detector is set to q_(high) in        correspondence of those time slots in which the intensity        modulation pattern presents a value u₁. In FIGS. 2 and 3 this        happens in correspondence of the 3^(rd) time slot. In all the        other time slots the gain is set to q_(low).    -   The same procedure is repeated for j=3 and all the other values        of j.

In the top part of FIG. 3, a random pattern for the intensity modulatoris given. Similarly to FIG. 2, it comprises 3 values of j: j=2, j=1 andj=0. The corresponding intensities prepared by the Intensity Modulatorare then u₂, u₁ and u₀ respectively, randomly distributed along thesample.

The corresponding pattern for the Gated Detector gain is depicted in thebottom part of FIG. 3. It is composed by 4 sequential Frames whoselengths are related to the Gated Detector integration time, i.e., to howlong the Gated Detector collects photons before providing the outputcurrent. In each Frame a different setting is arranged. In Frame 2, thesetting is as such as to measure the intensity in the class j=2, u₂. Itis followed by Frame 1, to measure U_(I), Frame 0, to measure u₀, andFrame 3, to measure the average value of u_(j), indicated by u_(avg). Inthis latter case, the gain of the Gated Detector is set always to thelowest value, q_(low). The Frames can be disposed in arrangementsdifferent from the one depicted in FIG. 3. For instance, it is possibleto measure Frame 0 before Frame 1; or it is possible to measure Frame 3after Frame 2 and again after Frame 1 and after Frame 0.

With the above technique, it is possible to access the different valuesu_(j). To see that, consider the preferred embodiment with 3 intensitiesu₂, u₁ and u₀, and suppose for instance that Alice wants to measure thevalue of u₂ only. From the frame j-avg she obtains a current

I

_(j-avg) =I _(dc) +vq _(low)(u ₂ p ₂ +u ₁ p ₁ +u ₀ p ₀).   (1)

Then, as explained, in the frame j=2 Alice can set the gain of the GatedDetector to q_(high) only in correspondence of the j=2 values of theintensity modulation pattern. As a consequence, the Gated Detector willoutput the following current:

I

_(j=) =I _(dc) +vq _(high) u ₂ p ₂ +vq _(low)(u ₁ p ₁ +u ₀ p ₀).   (2)

By subtracting Eq.(1) from Eq. (2) one obtains:

ΔI

₂ =

I

_(j=2) −

I

_(j-avg) =v(q _(high) −q _(low))u ₂ p ₂ =vq _(high) u ₂ p ₂.   (3)

In the last passage it is considered that in every practical settingq_(high) is about 3 orders of magnitude bigger than q_(low). From Eq.(3) it is simple to obtain u₂:

$\begin{matrix}{u_{2} \cong {\frac{{\langle{\Delta \; I}\rangle}_{2}}{v\; q_{high}p_{2}}.}} & (4)\end{matrix}$

All the quantities in the numerator of Eq. (4) are known to Alice andthe quantity in the denominator can be measured, so u₂ can be obtained.The same procedure can be repeated until all the other intensities u_(j)are determined.

In the above embodiments, the frame length is shown as 8 pulses.However, this is for illustrative purposes only. In some embodiments theframe lengths will be at least 10⁶ pulses. In other embodiments at least10⁸ pulses or at least 10⁹ pulses.

In some embodiments, the frame will last 1 second, with the sendersending at a rate of at least 10⁶ pulses per second. In furtherembodiments, longer or shorter frame lengths may be used.

In FIG. 2, a box is shown around the detection system 15. In someembodiments, the components are shielded so that one could not obtaininformation about the intensities sent by monitoring the gating signal.

In an embodiment, the control signal provided allows the intensitydistributions to be random.

FIG. 4 is a schematic of a sender 11 which is configured to handleBasis/Bit Selection. If the sending unit is used in a quantumcommunication system, the system may comprise a unit for selecting basis(Z, X) which defines the photon polarization, or its relative phase, byaligning it along the Z axis of the Poincare sphere or along the X axisof the Poincare sphere. This selection may be performed in a random way:the basis Z is selected with probability p_(z)(0≦p_(z)≦1) and the basisX with probability p_(x)=1-p_(z).

The system may also comprise a unit for selecting bit values (0, 1).Thus sets the value of the bit that Alice wants to transmit to Bob. Inan example, if a photon has its polarisation aligned along the Z axis,then a 0 or a 1 correspond to further aligning its polarisation alongthe positive or the negative direction of the Z axis, respectively; allthe same, if its polarisation was aligned along the X axis, then a 0 ora 1 correspond to further aligning it along the positive or the negativedirection of the X axis, respectively. The same holds true if thepolarisation is replaced by other degrees of freedom (DoF), like theabove-mentioned relative phase. The bit selection just described isusually effected by Alice in a random way and the bit 0 and 1 are chosenwith the probabilities p₀ and p₁ respectively.

When basis and bit selection are performed, the components used canmodulate the intensity of the pulses. Although it is not the intentionfor these components to modulate the intensity.

The system of FIG. 4 is similar to that of FIG. 2 but the component 17is replaced with Basis/Bit selection module 31. To avoid any unnecessaryrepetition, like reference numerals will be used to denote likefeatures.

The intensity of the light exiting the Basis/Bit Selection box depends(slightly) on the basis modulation pattern. This is unwanted behaviourbecause in QKD it is important to guarantee that the outgoing intensityis independent of the selected basis.

This time the intensities coming out of the Basis/Bit Selection boxwould explicitly depend on a basis label “b”. Such an effect should beavoided in QKD as it can cause an information leakage towards theeavesdropper. So it becomes important to measure each u_(b) separately,to confirm that there is no such effect.

In FIG. 4, the modulation pattern for the Gated Detector gain is asfollows:

-   -   a first pattern is prepared to measure the basis b=1. The        voltage on the Gated Detector is set to q_(high) in        correspondence of those time slots in which the basis modulation        pattern presents a value equal to 1. In FIG. 4 this happens in        correspondence of the 1^(st), 2^(nd), 4^(th) and 6^(th) time        slot. In all the other time slots the gain is set to “q_(low)”.    -   A second pattern is prepared to measure the basis b=0. The        voltage on the Gated Detector is set to q_(high) in        correspondence of those time slots in which the basis modulation        pattern presents a value equal to 0. In FIG. 4 this happens in        correspondence of the 3^(rd) and 5^(th) time slots. In all the        other time slots the gain is set to q_(low).    -   In addition to the Basis Selection, the same procedure can be        adopted to monitor the behaviour of the Bit Selection, by        providing the Gated Detector with a gain modulation pattern        correlated with the bit modulation pattern.

As far as the Basis Selection is concerned, the equations are similar tothose already provided in the case of the Intensity Modulation. Forexample, when b=1, we have:

I

_(b-avg) =I _(dc) +vq _(low)(u _(b=1) p _(b=1) +u _(b=0) p _(b=0)),  (5)

I

_(b=1) =I _(dc) +v(q _(high) u _(b=1) p _(b=1) +q _(low) u _(b=0) p_(b=0)).   (6)

By subtracting Eq. (5) from Eq. (6) we obtain:

ΔI

_(b=1) →v(q _(high) −q _(low))u _(b=1) p _(b=1) p _(b=1).   (7)

From Eq. (7) it is simple to obtain u_(b=1):

$\begin{matrix}{u_{b = 1} \cong {\frac{{\langle{\Delta \; I}\rangle}_{b = 1}}{v\; q_{high}p_{b = 1}}.}} & (8)\end{matrix}$

All the quantities in the numerator are known to Alice and the quantityin the denominator can be measured by her, so u₁ is accessible to Alice.

In the above embodiments, tight estimates for the u_(j) or the u_(b) andcan be applied in real time, during the very execution of QKD.

The above embodiments have used a gated detector. However, theembodiment of FIG. 5 uses a standard power meter. The components of thesystem of FIG. 5 are similar to those of FIG. 2 however the gateddetector of FIG. 2 has been replaced with a power meter 41 or other typeof light detector. To avoid unnecessary repetition, like referencenumerals have been used to denote like features.

The Power Meter 41 receives pulses from the beam splitter (23) and isused to monitor any fluctuation occurring to the light intensity sentinto the quantum channel. However, the Power Meter 41 measures theaverage intensity of the light impinging on it, <u_(j)> and not theintensity of the light in each class, u_(j). This happens because thePower Meter does not have access to the class index “j”. In anembodiment the Power Meter will have a long integration time andcollects all the light pulses impinging on it for the whole duration ofsuch a time interval. At the end of the acquisition period, the PowerMeter outputs a signal which results in the average light intensityreceived to the controller. Thus, the resulting outcome from the PowerMeter is actually averaged over all the possible values of j. Theaverage current from the Power Meter will then have the following form:

$\begin{matrix}{{\langle I\rangle}_{j} = {I_{dc} + {v\; q{\sum\limits_{j = 0}^{J}{u_{j}{{p\left( u_{j} \right)}.}}}}}} & (9)\end{matrix}$

I_(dc) is the dark count current; v is the repetition rate of the lightpulses; q is a multiplicative factor, later on called the “detectorgain”, providing the proper dimensions to the problem; J is the totalnumber of light intensities used in the protocol; u_(j) is the intensityof the pulse in the class j ; p(u_(j)) is the “class probability”, i.e.the probability that a pulse in the class “j” is prepared by Alice. Inan embodiment of where there are 3 different intensities prepared by theIntensity Modulator, Eq. (9) is explicitly rewritten as:

$\begin{matrix}\begin{matrix}{{\langle I\rangle}_{j} = {I_{dc} + {v\; {q\left\lbrack {{u_{2}{p\left( u_{2} \right)}} + {u_{1}{p\left( u_{1} \right)}} + {u_{0}{p\left( u_{0} \right)}}} \right\rbrack}}}} \\{= {I_{dc} + {v\; {q\left( {{u_{2}p_{2}} + {u_{1}p_{1}} + {u_{0}p_{0}}} \right)}(11)}}}\end{matrix} & (10)\end{matrix}$

where in Eq. (9) p(u_(j))=p_(j). In the above equations, I_(dc) andu_(j) are unknown. In general, the dark current I_(dc) can be measuredin a separate experiment, or can be learnt from the factoryspecifications of the Power Meter. However, the problem tends to reduceto Eq. (9), and 2 or more unknowns, e.g. u₂, u₁ and u₀. Hence theproblem is underdetermined and it is not possible to solve it in orderto obtain the value of each u_(j) separately.

In the system of FIG. 5, equation (9) may be solved by varying the classprobabilities p_(j), as schematically depicted in FIG. 5. In thissetting, the detector gain is no more modulated and will be simplyindicated with the label “q”.

Considering again the preferred embodiment with the 3 values u₂, u₁ andu₀. Alice chooses each class j at random, with probabilityp_(j)=p(u_(j)). As an example, p₂ can be about 90%, p₁ about 9% and p₀about 1%, but different values are possible. In this case the currentoutputted by the Power Meter is the same as in Eq. (11). This is calledthe “Frame A current”, with reference to FIG. 5 and to FIG. 3. Inparticular, the length of the Frame is defined by the Power Meterintegration time, as in FIG. 3, while the label “Frame A” comes fromFIG. 5 and refers to the particular choice of the triple (p₂, p₁, p₀).The Frame A current is given by:

I

_(FrameA) =I _(dc) +vq(u ₂ p ₂ +u ₁ p ₁ +u ₀ p ₀).   (12)

Alice can then choose two additional Frames in order to obtain two moreequations for her linear system. When Frame B and C are chosen, thefollowing currents are generated by the Power Meter:

I

_(FrameB) =I _(dc) +vq(u ₂ p′ ₂ +u ₁ p′ ₁ +u ₀ p′ ₀)   (13)

I

_(FrameC) =I _(dc) +vq(u ₂ p″ ₂ +u ₁ p″ ₁ +u ₀ p″ ₀)   (14)

With the additional knowledge of the dark current I_(dc), Alice has nowthree equations in three unknowns and she can solve the problem for eachu_(j).

There are two constraints in the choice of the p_(j). The first is thecompleteness condition:

$\begin{matrix}{{\sum\limits_{j}p_{j}} = 1.} & (15)\end{matrix}$

The second is the fact that the lines of the various probability setsmust be linearly independent. This can be expressed through the rank ofa probability matrix:

$\begin{matrix}{{{{rank}\begin{pmatrix}p_{2} & p_{1} & p_{0} \\p_{2}^{\prime} & p_{1}^{\prime} & p_{0}^{\prime} \\p_{2}^{''} & p_{1}^{''} & p_{0}^{''} \\\ldots & \; & \;\end{pmatrix}} \geq J},} & (16)\end{matrix}$

where J is the maximum number of settings of the intensity modulator:j={0,1,2, . . . , J}.

The same technique can be used to monitor and or calibrate the boxBasis/Bit Selection. In this case the starting equation is Eq. (5), inwhich the two probabilities p_(b=1) and p_(b=0) appear explicitly. Alicecan choose two different Frames for the probabilities p_(b=0) andp_(b=1) so to obtain a system of two equations in two unknowns and solveit again, so to get each u_(b) separately.

The above embodiments allow the use of a standard power meter andrequire little calibration.

In the embodiments described with reference to FIGS. 2 to 5, a feedbackloop may be provided. For example, if the controller is provided withinformation concerning the required intensities outputted by thecomponent, the controller can increase or decrease the control signal asnecessary depending on the intensity measured by the detection system.Thus, the above embodiments allow a calibration system to be introducedto a quantum communication system. Although the above embodiments haveshown one detection system, multiple detection systems can be providedfor each component which is likely to affect the intensity. Also,multiple components may be monitored using a single detection system.

The above systems avoid problems due to unexpected deviations inintensity due to components which either intentionally orunintentionally vary the intensity. Thus the above system can avoidhidden correlations happening between bit/basis selection and intensityfluctuations.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed the novel methods and apparatusdescribed herein may be embodied in a variety of other forms;furthermore, various omissions, substitutions and changes in the form ofmethods and apparatus described herein may be made without departingfrom the spirit of the inventions. The accompanying claims and theirequivalents are intended to cover such forms of modifications as wouldfall within the scope and spirit of the inventions.

1. A detection system for monitoring the intensity of a stream ofmodulated pulses, the system comprising a controller configured toprovide a control signal to a component outputting modulated pulses, thecontrol signal controlling the level of modulation for each pulseexiting the component and a detector configured to measure the intensityof the pulses in the stream of pulses exiting the component outputtingmodulated pulses, wherein the detector comprises a gated detector, thecontroller being configured to send a gating signal to said detector,wherein said gating signal varies the gain of the detector with thecontrol signal such that pulses with a selected modulation level can bedetected.
 2. A detection system according to claim 1, wherein thecontroller is configured to group the pulses into frames with each framecomprising a plurality of pulses, the controller being configured tocontrol the gain of the detector to detect pulses modulated by aselected control signal in each frame
 3. A detection system according toclaim 2, wherein the controller is configured to control the componentto vary the modulation between J different levels where J is an integerof at least 1, the controller being configured to control the detectorto measure over at least J frames, such that the gain of the detector isset to detect pulses of intensity level J for each of the J frames.
 4. Adetection system according to claim 2, wherein the output of thedetector is integrated over each frame.
 5. A detection system accordingto claim 1, wherein the detector is an avalanche photodiode.
 6. Adetection system according to claim 2, wherein a frame comprises atleast 10² pulses.
 7. A detection system according to claim 1, whereinthe controller is configured to vary the control signal sent to thecomponent outputting modulated pulses dependent on the measuredintensities.
 8. A sending unit for a quantum communication system,comprising a source of pulses and the detection system of claim
 1. 9. Asending unit according to claim 8, wherein the component outputtingmodulated pulses is selected from a component configured to set thebasis or bit value of the pulses, an intensity modulator and a variableintensity source of pulses.
 10. A detection system for monitoring theintensity of a stream of modulated pulses, the system comprising acontroller configured to provide a control signal to a componentoutputting modulated pulses and a detector configured to measure theintensity of the pulses in the stream of pulses exiting the componentoutputting modulated pulses, the controller being configured to groupthe pulses into frames and control the detector to measure the averageintensity of each frame, the controller being configured to apply acontrol signal selected from J different modulation levels where J is aninteger of at least 1 and vary the distribution of the pulses betweenthe frames such that J frames can be produced with a differentdistribution pulses subjected to the control signals in each frame, thecontroller being configured to associate the distribution of pulses withthe detected average intensity value for each frame.
 11. A detectionsystem according to claim 10, the controller further comprising acalculating unit, the calculating unit being configured to receiveinformation concerning the distribution of pulses in a frame with thedetected average intensity for each frame and calculate the intensity ofthe pulses exiting the modulating component for each control signalapplied by the controller from:${\langle I\rangle}_{j} = {I_{dc} + {v\; q{\sum\limits_{j = 0}^{J}{u_{j}{p\left( u_{j} \right)}}}}}$Where <I>j is the average intensity over a frame of pulses, I_(dc) isthe dark count current; v is the repetition rate of the light pulses; qis the “detector gain”, J is the total number of levels of controlsignals applied to the modulating component, u_(j) is the intensity ofthe pulse exiting the modulating component controlled by signal of levelj; and p(u_(j)) is the probability that level j was applied to themodulator when the pulse passed through the modulating component.
 12. Adetection system according to claim 10, wherein the frames comprise atleast 10² pulses.
 13. A detection system according to claim 11, whereinthe controller is configured to vary the control signal sent to themodulating component dependent on the measured intensities.
 14. Asending unit for a quantum communication system, comprising a source ofpulses and the detection system of claim
 10. 15. A sending unitaccording to claim 14, wherein the component outputting modulated pulsesis selected from a component configured to set the basis or bit value ofthe pulses, an intensity modulator and a variable intensity source ofphotons.
 16. A method of monitoring the intensity of a stream ofmodulated pulses, the method comprising: outputting a stream ofmodulated pulses from a component; providing a control signal to saidcomponent to modulate said pulses, the control signal controlling thelevel of modulation applied to each pulse by the component; andmeasuring the intensity of the pulses exiting the component using agated detector, said detector being controlled using a gating signalwhich varies the gain of the detector such that pulses with a selectedmodulation level can be detected.
 17. A method of monitoring theintensity of a stream of modulated pulses, the method comprising:outputting a stream of modulated pulses from a component; providing acontrol signal to said component to modulate said pulses, the controlsignal selected from J different modulation levels where J is an integerof at least 1; grouping the pulses into J frames wherein thedistribution of the pulses with different modulation levels is variedbetween the frames; and measuring the average intensity of each of theframes and associating the distribution of pulses with the detectedaverage intensity value for each frame.